The present invention relates to a liquid crystal display comprising a first substrate and a second substrate opposed to each other, and liquid crystal charged between the first and the second substrates.
The most prospective one of the flat panel displays which are able to replace CRT displays is TFT-LCDs (Thin Film Transistor Liquid Crystal Displays). The TFT-LCDs are expected to expand their market by being applied in civil equipment, such as personal computers, word processors, OA (Office Automation) equipment, portable televisions, etc., and domestic electric equipment.
The fabrication technique of TN (Twisted Nematic) TFT-LCDs has made remarkable progress to have eventually surpassed CRT displays in properties of contrast, color reproducibility, etc.
The currently most popular normally white mode-TN liquid crystal display will be explained with reference to FIG. 42. FIG. 42A shows the state of the display with no voltage applied, and FIG. 42B shows the state of the display with a voltage applied.
An about 5 .mu.m-thickness TN liquid crystal layer 200 is sandwiched by a pair of glass substrates 202, 204. An orientation direction of oriented films of the glass substrates 202, 204 are offset by 90.degree. with respect to each other. The TN liquid crystal layer 200 is oriented along the orientation directions of the oriented films of the glass substrates. 202, 204, and, as shown in FIG. 42A, a direction of liquid crystal molecules are twisted by 90.degree.. A polarizing plate 206 is disposed outside the glass substrate 204 in parallelism with the orientation direction of the glass substrate 202. A polarizing plate 208 is disposed outside the glass substrate 204 in parallelism with the orientation direction of the glass substrate 204.
In the state in which no voltage is applied to the TN liquid crystal layer 200, as shown in FIG. 42A, light incident on the TN LCD passes the polarizing plate 206 to be linearly polarized light, passes the glass substrate 202 to be incident on the TN liquid crystal layer 200, and is twisted by 90.degree. along the orientation of the TN liquid crystal layer 200. The light which has passed the TN liquid crystal layer 200 passes the glass substrate 204 and passes the polarizing plate 208. The display at this time is bright.
When a voltage is applied to the TN liquid crystal layer 200, as shown in FIG. 42B, the twist of the liquid crystal molecules in the liquid crystal layer 200 vanishes. Light incident on the TN liquid crystal layer 200 advances without twist of the polarizing direction and is blocked by the polarizing plate 208. The display at this time is dark.
Thus, the TN liquid crystal display can control brightness of displays.
However, the TN liquid crystal display has a disadvantage of small field angles. To eliminate this disadvantage, improvements have been proposed using 1) phase difference film, 2) diffusion film, 3) orientation division, 4) random orientation, 5) in-plane switching (IPS), etc.
In the proposal using phase difference film, anisotropy of a refractive index due to an orientation direction of the liquid crystal molecules is compensated by phase difference film. However, this proposal cannot sufficiently improve field angles.
In the proposal using diffusion film, light which has passed the liquid crystal display is diffused to substantially obtain a wider view angle. However, in this proposal widening the view angle lowers display resolution.
In the proposal using orientation division, the liquid crystal is divided in a number of tiny domains, and the tiny domains have different orientations of the liquid crystal molecules to mutually compensate view angle characteristics by the domains. However, this proposal cannot sufficiently improve field angles.
In the proposal using random orientation, an amorphous TN liquid crystal is used to form at random a number of tiny domains having different orientations. However, this proposal does not sufficiently improve the view angle either.
In the proposal using in-plane switching (IPS), two drive electrodes apply an in-plane voltage to one of a pair of substrates sandwiching the liquid crystal, and the voltage application to the drive electrodes is controlled, whereby orientation of the liquid crystal molecules is controlled.
This proposal has been long known as a structure of the liquid crystal displays but is noted because of its good field angle characteristics (Japan Display 95 Digest, p. 707, 1. 995). In this in-plane switching, the liquid crystal molecules change orientation, keeping horizontal to the surfaces of the substrates. Thus the in-plane switching is principally superior in visual characteristics.
However, in the in-plane switching (IPS), two drive electrodes for the voltage application are disposed on one of a pair of substrates sandwiching the liquid crystal, and a resulting short circuit tends to occur between the adjacent electrodes, which makes it difficult to ensure high yields in the fabrication of liquid crystal displays of high precision picture elements.
As shown in FIG. 43A, the drive voltage heavily depends on the thickness of the liquid crystal. For a liquid crystal to be driven, it must have a thickness smaller than a certain thickness. As shown in FIG. 43B, a drive voltage heavily depends on a gap between the electrodes. To drive the liquid crystal, it is necessary to provide a gap larger than a certain value, which hinders micronization.
Since the two drive electrodes are formed on the other substrate, liquid crystal displays having high precision picture elements have extremely low aperture ratios.